The synthetic resin formed by the polymerization of propylene as the sole monomer is called polypropylene. While "polypropylene" has been used from time to time in the art to include a synthetic resin formed by the copolymerization of propylene and a minor amount of another monomer, such as ethylene, the term is not so used herein. The resin generally is a mixture of propylene homopolymers of different chain lengths and thus different molecular weights. However, it usually is referred to simply as "polymer".
The well-known polypropylene of commerce is a predominantly isotactic, predominantly crystalline, thermoplastic polymer. It is formed by the polymerization of propylene by Ziegler-Natta catalysis. As is well known, in such catalysis the polymerization catalyst comprises the reaction product of an organic compound of a metal of Groups I-III of the Periodic Table (for example, an aluminum alkyl), and an inorganic compound of a transition metal of Groups IV-VIII of the Periodic Table (for example, a titanium halide). The polymerization conditions generally are well known also.
Such polypropylene has achieved tonnage use in many applications, one of which is the manufacture of film, especially biaxially oriented film used in the packaging of, for example, snack foods such as potato chips and the like. Indeed, such film has become known as "BOPP" film.
Oriented polypropylene film generally is made by a method comprising solid state orientation. One such method is the tubular blown or double bubble method. Another is the tenter frame method.
In one embodiment of the tubular blown or double bubble method, a composition consisting essentially of polypropylene is melt extruded through an annular die to form a molten tube which is brought into contact with an internal, cooling mandrel where it begins to solidify. The tube is further cooled in a water bath, and pulled through a pair of rolls which compress it tightly. After passing through the rolls, the cooled, solidified tube is "softened" by reheating with, for example, radiant heaters to a temperature below the melting point of the polypropylene, but high enough to enable the tube to be expanded by internal air pressure. The tube is insufflated by air under pressure. This causes the tube to enlarge and form a controlled bubble, thereby stretching the wall of the tube in all directions, and thereby biaxially orienting it. The bubble then is collapsed through frame rollers, between a pair of nib rolls, and passed through a slitter where the flattened bubble is slit into a strip or strips which is or are wound on a take-up reel or take-up reels.
In the tenter frame method a composition consisting essentially of polypropylene is melt extruded through a slot die to form a molten sheet. The sheet is solidified by quench cooling onto a cast roll. In the on-line orientation practice of this method, the resulting cast sheet is immediately reheated to a temperature below the melting point of the polypropylene, but high enough to enable the composition to be drawn or stretched. In the case of sequential orientation, the thus "softened" cast sheet is drawn by rolls rotating at different speeds of rotation such that the sheet is stretched to the desired draw ratio in the longitudinal direction (machine direction). The thus uniaxially oriented sheet is subsequently clamped at its lateral edges by chain clips and conveyed into a tenter oven where it is again reheated to such a temperature, and drawn in the transverse direction to the desired draw ratio by means of the chain clips which are diverged in their forward travel. The resulting biaxially oriented film is released from the clips, edge-trimmed, and wound on a take-up reel.
Under development in recent years has been the polymerization of l-olefins by metallocene catalysis. In fact, the development of such has reached the stage at which at least one polypropylene producer has made commercially available samples of predominantly isotactic polypropylene produced by the metallocene catalyzed polymerization of propylene. In such catalysis the polymerization catalyst comprises the reaction product of a metallocene and a compound such as an alumoxane, trialkyl aluminum or an ionic activator. A metallocene is a compound with at least one cyclopentadienyl moiety in combination with a transition metal of Groups IV-VIII of the Periodic Table.